1. Field of the Invention
This invention related to internal combustion engines which employ flexible link transmissions, such as belts. The belts rely on mechanical adherence, or friction, to drive a pulley. Such a belt operates driven devices which receive mechanical power, via the belt which is driven by an internal combustion engine. The belts are used, more specifically, to drive accessories on automobile engines. The belts must be placed under a specific amount of tension for proper operation, and this tension may be applied by a roller positioned at the end of a movable lever arm. The lever arm is generally a part of a tensioning device which receives an elastic torque that can be provided by biasing means.
The biasing means may be an elastomeric sandwich which is integrally bonded to rigid armatures, and thereby utilizes the elastic force from the shearing of the elastomeric sandwich to ensure that the engagement of the transmission belt with the pulleys is maintained.
2. Background Information
Flexible link transmissions which operate by mechanical adherence of a flexible link to pulleys, require an elastic tensioning means to make possible the mechanical adherence of the flexible link to the pulleys. This is essentially true whether the driving surface of the flexible link is flat, trapezoidal or grooved. Such a flexible link is commonly called a "belt", and these belts are generally made of cabled materials having high rigidity, such as polyester, glass fiber or aramid.
Such flexible links, or belts are used on automobiles, or stationary engines to drive accessories such as, water pumps, alternators, air conditioners, power steering pumps, etc. These accessories are positively driven by the flexible link which connects the accessories to a driven shaft of the engine The adherence of the belt to the pulleys of the driven accessories and to the driven shaft is generally maintained by a tensioning device which utilizes an idler pulley.
The term "idler pulley" is commonly used when the high intrinsic rigidity of the longitudinal armature of the belts, requires a permanent tension established by an elastic thrust on a guide pulley. Such a guide pulley is most conveniently arranged in contact with the smooth surface constituting the back of the belt. By using such an arrangement, any variations caused by thermal expansion of the various constituents of the system, and primarily all the momentary elastic variations or permanent modifications of the length of the perimeter of the belt due to wear or to creep, even if these variations are low, are compensated for by displacement of the elastic tensioning means of the idler pulley.
There are numerous types of idler pulleys which operate by the pivoting of an arm around an axis, which arm is elastically tensioned by a metal spring, with or without damping means. Tensioning devices which integrate both of the two functions of tensioning and damping into an elastomeric ring are disclosed in U.S. Pat. No. 4,144,772 (The Toro Company), in GB 2,070,724 (Alan Cresley Pritchard) and EP 0,157,193 (RIV-SKF). The above patents all have in common an elastic recall which is provided by torsion of an elastomeric ring. The devices of these patents, because of the provision of their pivoting on an elastomer material, also have in common the disadvantage that such pivoting also makes possible spherical conical tipping, or wobbling, the angular variations of which interfere with the centering of the belt on the pulley of the tensioning device.
This tipping situation can be improved by using a ring having a longer longitudinal length, which ring is integral with concentric and prestressed armatures. Such a device is disclosed in U.S. Pat. No. 3,975,965 (Dayco Corporation) and EP 0,114,779 (Caoutchouc Manufacture et Plastiques), which devices differ in terms of the tension regulation means, the latter being provided by the rotation of the internal ring. In these two devices the damping, which remains relatively low, is provided exclusively by the visco-elastic character of an elastomer. In addition, Caoutchouc Manufacture et Plastiques, the assignee of the present application, proposed in its French patent application No. 2,640,016, which corresponds to U.S. Pat. No. 5,033,423, a much more radical remedy to the risks of conical tipping in relation to the axis of rotation. This remedy provides the return torque by the deformation of a planar, prestressed elastomeric sandwich, and also provides the damping of dynamic variations by friction of a washer subjected to the stated prestress. Thus, the tipping torques are applied via solids in sliding contact, under much better conditions than via a layer of elastomer, no matter what the level of prestress applied to the device.
This operation is essentially the same as the operation of the device disclosed in patent CH 0,318,686 (Baechen), in which rubber sheets are prestressed between two plates. In both of the devices described above, the dynamic displacements are recalled solely by the rigidity of an elastomeric element, and the damping is provided with a constant force by the rubbing of surfaces fitted with appropriate friction linings.
An asymmetrical damping, in the form of friction, is the subject of documents FR 2,591,700 and FR 2,611,018 (SKF), EP 0,294,919 (Litens Automotive) and DE 3,637,103 (INA Walzlager). A hydraulic damping which is also asymmetrical is provided by the device disclosed in FR 2,617,556 (Hutchinson).
In all of these devices, the tension is provided by metal springs, but each of these arrangements show the requirement for asymmetry in the dynamic variation of tension, which should preferably be more rigid during the increase of the tension, and which should also preferably be more flexible during the relaxation of the tension The predetermined values for this type of damping, whether frictional or hydraulic, although asymmetrical, do not solve all the problems resulting from ranges of spring deflections of variable amplitudes.
An analysis of the prior art shows that none of the devices of the prior art, whatever the type of damping used, achieve an elastic recall having a double rigidity which is asymmetrical on the two sides of the most common service position. However, such a double rigidity is desirable, even essential, in applications involving engines having a lightweight flywheel where the engine pulses can exceed the average value of the drive force of the accessories, i.e., in Diesel engines. Because of these pulses, the normally relaxed strand of the flexible link on which the idler pulley is located, suddenly can become taut and the tensioning device must be able to exert a significantly higher rigidity to prevent a sudden relaxation of the other strands of the flexible link.